Superconductor-polymer composites

ABSTRACT

Superconductor-polymer composite materials comprise a matrix formed of a  rmoplastic polymer and a superconductor powder dispersed in the matrix. The superconductor powder preferably has a composition RBa 2  Cu 3  O 7-x  wherein R is a rare earth metal and x is less than or equal to 1. The thermoplastic polymer matrix comprises a vinylidene fluoride homopolymer or copolymer. The composite materials may be formed as shaped products, sheets or films.

FIELD OF THE INVENTION

The present invention relates to superconductor-polymer compositematerials and to methods of forming shaped composite materials andcomposite sheet materials. More particularly, the invention relates tosuperconductor-polymer composite materials comprising a matrix formed ofa thermoplastic polymer and a superconductor powder dispersed in thematrix.

BACKGROUND OF THE INVENTION

It has recently been discovered that certain mixed metal oxide materialsexhibit high-temperature superconductivity. See, for example, Wu et al,Physical Review Letters, Vol. 58, No. 9, pages 908-910 and Moodenbaughet al, Physical Review Letters, Vol. 58, No. 18, pages 1885-1887.Specifically, various mixed metal oxide materials comprising rare earthmetal oxides, alkaline earth metal oxides and copper oxides have beendiscovered to exhibit high temperature superconductivity. The rare earthoxide-alkaline earth oxide-copper oxide superconductor materials aregenerally of the formula

RBa₂ Cu₃ O_(7-x) where x is less than or equal to 1. Because thesuperconducting transition temperature T_(c) of such materials isgreater than 77° K., these mixed oxide materials have an increasingnumber of promising technological applications.

However, the ceramic nature of the superconductor materials poses anumber of problems for the manufacture of high T_(c) superconductingshaped products such as magnetic levitation components and magneticshielding devices. Additionally, because the mixed oxide superconductingmaterials are susceptible to degradation by moisture and chemicals suchas reducing agents, their use in various applications is limited.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to providesuperconductor composite materials which facilitate the cost effectivemanufacture of high T_(c) superconducting products of various shapes. Itis an additional object of the invention to provide superconductorcomposite materials which are resistant to degradation of thesuperconducting properties thereof owing to exposure to moisture andchemicals such as reducing agents. It is a further object of theinvention to provide a superconductor composite material which issuitable for use in preparing magnetic levitation components, magneticshielding devices and the like.

These and additional objects are provided by the superconductor-polymercomposite materials and methods according to the present invention. Thecomposite materials of the invention comprise a matrix formed of athermoplastic polymer and a superconducting powder dispersed in thematrix. The superconductor powder preferably has a composition RBa₂ Cu₃O_(7-x) wherein R is a rare earth metal and x is less than or equalto 1. According to the methods of the present invention, the compositematerials may be formed as shaped products, sheets or films andtherefore facilitate the manufacture of high T_(c) superconductingdevices of various shapes. Additionally, the polymeric matrix protectsthe mixed oxide superconductors from exposure to deleteriousenvironmental effects, for example, moisture and chemicals such asreducing agents.

These and additional objects and advantages will be more apparent inview of the following detailed description.

DETAILED DESCRIPTION

The superconductor-polymer composites according to the present inventioncomprise a matrix formed of a thermoplastic polymer and a superconductorpowder dispersed in the matrix. The polymer matrix acts as a binder forthe superconductor powder and is chemically nonreactive with thesuperconductor powder in order that the superconducting properties ofthe superconductor powder are not degraded. It is important that thethermoplastic polymer is impervious to moisture and to chemicals presentin the atmosphere or environment during use of the composite material inorder to protect the chemically delicate superconductor powder. Thepolymer matrix must also exhibit suitable thermal and mechanicalproperties which allow it to withstand repeated cooling and warmingbetween room temperature and cryogenic temperatures during its servicelife. It is also desirable that the thermoplastic polymer has roomtemperature flexibility and therefore that its glass transitiontemperature is less than about 25° C. In many applications, it isdesirable that the glass transition temperature of the polymer matrix iswell below room temperature, for example, as low as -40° C.

In a preferred embodiment, the thermoplastic polymer which forms thematrix of the polymer material comprises a vinylidene fluoridehomopolymer or copolymer. Such polymers are preferred in that theyexhibit low glass transition temperatures, suitable thermal andmechanical properties and good toughness. In addition, the vinylidenefluoride polymers exhibit good mold release properties and favorableextrusion properties which permit processing in convenient forms such asrolls of sheet material or film, wires, tapes and molded shapes. Bothhomopolymers and copolymers of vinylidene fluoride are preferred for usein the composite materials of the present invention. Vinylidene fluoridecopolymers may be formed of one or more comonomers such asfluorinecontaining olefins copolymerized with vinylidene fluoride.Suitable fluorine-containing olefins include vinyl fluoride,trifluoroethylene, chlorofluorovinylidene, trifluorochloroethylene,tetrafluoroethylene and hexafluoropropylene. Preferred comonomerscomprise tetrafluoroethylene and trifluoroethylene. Additionally, thevinylidene fluoride homopolymers or copolymers may be blended with oneor more additional polymers for use in the matrix of the compositematerial of the invention. Examples of additional polymers suitable forblending with vinylidene fluoride homopolymers or copolymers includeacrylate polymers, methacrylate polymers and vinyl chloride polymers. Apreferred thermoplastic blend for use as the matrix comprises ahomopolymer or copolymer of vinylidene fluoride with poly(ethylmethacrylate).

The superconductor powder which is dispersed in the thermoplasticpolymer matrix is a mixed oxide material which exhibits thesuperconducting Meissner effect at temperatures greater than 77° K. Thesuperconductor material is in powder form and preferably has acomposition RBa₂ Cu₃ O_(7-x) wherein R is a rare earth metal and x isless than or equal to 1. Other superconducting materials which exhibitthe Meissner effect at temperatures greater than 77° K. are alsosuitable for use in and within the scope of the present invention.Preferably, the superconductor powder comprises YBa₂ Cu₃ O_(7-x) where xis less than or equal to 1.

A wide range of loading of the superconductor powder component may beused in the composite materials of the present invention. Inapplications such as magnetic levitation, the required diamagneticfunction of the composite material does not require electricalcontinuity from one superconducting powder particle to another so thatlower loadings of superconducting powders may be used. The amount ofsuperconductor powder included in the composites determines themechanical properties and the effective magnetic susceptibility of thecomposite. The effective magnetic susceptibility may be anywhere between0 and -1, values close to -1 corresponding to composite materialscontaining close to 100 percent superconductor powder and values closeto 0 corresponding to composite materials containing close to 0 percentsuperconductor powder. Preferably, the composite materials according tothe present invention comprise from about 20 to about 80 weight percentsuperconductor powder.

The composite materials according to the present invention may furtherinclude conventional additives for their intended purposes, for example,mold-release agents, chemical agents and the like.

The present invention also relates to methods for forming shapedproducts, sheets or films comprising the composite materials. In a firstembodiment of the present method, the composite material is formed intoa shaped product by pressure molding powdered mixtures of thethermoplastic polymer and the superconductor. For example, asuperconductor powder such as YBa₂ Cu₃ O_(7-x) having a T_(c) of 90° K.is ground into powder form and thoroughly mixed with polyvinylidenefluoride powder having a melting temperature of approximately 450° K.The resulting mixture is introduced into a shaped mold and heated up tothe melting point of the polymer powder so that the polymer powdergrains fuse together and bind to the superconductor powder grains orimmobilize the superconductor powder grains therein without necessarilybinding thereto. The mold is then allowed to cool and the moldedcomposite material is removed. During the heating step, the mixture mayalso be subjected to a compacting pressure so that the mixture iscompacted at the same time that it is heated. Magnetic levitationcomponents in shapes ranging from simple pads to complicated structuresmay be fabricated in this manner.

In a second embodiment of the present method, the mixed powders of thepolymer and the superconductor are hot pressed and fused into a flatsheet. After cooling to room temperature, the sheet is flexible and maybe rolled into a shape, for example, a cylindrical shell, or formed bypressure by further heating, or both, into more complex shapes such asspherical or dome-shaped products. A plurality of shells of varyingdimensions formed according to this method may be assembled to form amultiple layered enclosure such as a series of coaxial cylindricalshells or concentric spherical shells. Such enclosures, when cooledbelow the superconducting transition temperature T_(c) constitutemagnetic shields because of the flux exclusion properties of thesuperconductor particles and their collective response to externalmagnetic fields. This collective response is equivalent in effect to theresponse of a homogeneous medium with an effective negative magneticsusceptibility.

The composite materials according to the present invention thereforeallow the use of superconducting materials in an increased number oftechnological applications. The preferred embodiments set forth aboveare intended to illustrate specific embodiments of the invention and arenot intended to limit the scope of the compositions and methods of thepresent invention. Additional embodiments and advantages within thescope of the claimed invention will be apparent to one of ordinary skillin the art.

What is claimed is:
 1. Composite material, comprising a matrix formed ofa thermoplastic polymer and a superconductor powder dispersed in thematrix, the superconductor powder having a composition RBa₂ Cu₃ O_(7-x)wherein R is a rare earth metal and x is less than or equal to 1, saidthermoplastic polymer comprising a vinylidene fluoride homopolymer orcopolymer.
 2. Composite material as defined by claim 1, wherein thethermoplastic polymer exhibits a glass transition temperature less thanabout 25° C.
 3. Composite material as defined by claim 1, wherein thethermoplastic polymer comprises a copolymer of vinylidene fluoride and amonomer selected from tetrafluoroethylene and trifluoroethylene. 4.Composite material as defined by claim 1, wherein the matrix furtherincludes an acrylate or methacrylate polymer.
 5. Composite material asdefined by claim 1, wherein the superconductor powder comprises YBa₂ Cu₃O_(7-x) where x is less than or equal to
 1. 6. Composite material asdefined by claim 1, wherein the superconductor powder is included in anamount of from about 20 to about 80 weight percent.
 7. Compositematerial, comprising a matrix formed of polyvinylidene fluoride and asuperconducting powder of the composition YBa₂ Cu₃ O_(7-x) where x isless than or equal to 1, dispersed in the matrix.
 8. Composite material,comprising a matrix formed of a thermoplastic polymer and asuperconductor power dispersed in the matrix, the superconductor powdercomprising a mixed metal oxide amterial which exhibits thesuperconducting Meissner effect at temperatures greater than 77° K., thethermoplastic polymer comprising a vinylidene fluoride homopolymer orcopolymer.